Lithographic printing, which is a type of planographic printing, is a well known and established art. In general, the process involves printing from a flat plate or cylinder having substantially no surface relief (hence, the term "planographic"), and depends upon different properties of the image and non-image areas of the surface for printability. In lithography, the image to be reproduced is imparted to the plate by any one of several methods well known to those skilled in the art in such a way that the non-image areas are rendered hydrophilic while the image areas are hydrophobic. A widely practiced technique employs a photosensitive coating for this purpose. Following exposure of the photosensitive coating to imagewise modulated light, the latent image is developed and a portion of the coating is removed from the plate. Next, the plate is treated with a desensitizing solution to render the plate hydrophilic in the areas from which the photosensitive coating has been removed. During the actual printing process, an aqueous fountain solution is applied to the plate surface. The fountain solution keeps moist all portions of the surface not covered by the hydrophobic image. Furthermore, the fountain solution prevents the plate from scumming, i.e. it prevents the non-image areas from becoming at least partially ink-receptive. The fountain solution may be formulated to gradually etch the surface of the plate just enough to keep the lines sharp and prevent rapid wear. In a conventional system, the fountain solution is applied to the plate by one or more rollers. At least one ink roller coated with an oil-based printing ink then contacts the entire surface of the plate but deposits the lithographic ink only on the image area since the hydrophilic non-image areas repel the ink. Hence, for each impression made during a run, the lithographic plate is first dampened with the aqueous fountain solution and then inked with a lithographic ink. Alternatively, the fountain solution and at least a portion of the oil-based ink are applied to the plate simultaneously with a first roller. In this latter system, other rollers, usually smaller in diameter than the first, may contact the plate subsequently to distribute the ink more evenly. Finally, the ink image is transferred directly to a paper sheet or other receptive surface to be printed, or to an offset blanket of rubber or synthetic material which in turn transfers the print to the final copy surface.
Gum arabic has long been used in aqueous solutions (both acidic and alkaline) in the preparation of lithographic plates. Gum arabic has been used, for example, in solutions for developing a latent image; as a desensitizing ingredient in a gumming solution--sometimes in combination with an etchant, in which case the solution is referred to as a gum etch--to make the non-image areas sharply defined and ink repellent, i.e. hydrophilic rather than hydrophobic; in a fountain solution, again sometimes in combination with an etchant, to keep the non-image areas hydrophilic during the press run; as a protective coating during idle periods on the press or even during storage for weeks and months; and in plate cleaner formulations.
Gum arabic is a natural product obtained as an exudate from acacia trees. The disadvantages of gum arabic are well recognized in the trade and a suitable substitute has long been sought.
Some of the compositions heretofore proposed as gum arabic substitutes for the treatment of image-bearing plates include oxalic acid (U.S. Pat. No. 3,489,561), homopolymers and copolymers of itaconic acid (U.S. Pat. No. 3,507,647), sodium carboxymethycellulose (U.S. Pat. No. 3,166,421), the copolymer of methyl vinyl ether and maleic anhydride (ibid.), polyacrylic acid (U.S. Pat. No. 3,211,686) and sodium and ammonium salts thereof (Japanese Patent Publication No. 49-6561 (1974), alginic acid and the sodium salt thereof, polyvinyl alcohol (U.S. Pat. No. 3,607,255), and polyacrylamide-based polymers, optionally blended with polyacrylic acid (U.S. Pat. No. 4,200,688). Gum arabic continues to be the composition of choice in industry, notwithstanding all of its disadvantages.